(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming a fuse as an electronic protection circuit by using polymeric films.
(2) Description of the Prior Art
Many if not most of the design and research activities in the field of semiconductors are directed at better methods of design and material applications related to major functions within the semiconductor devices. Therefore not less important is an aspect of semiconductor circuit design that relates to fusing of devices or device areas that are prone to be damaged by surges in voltage or current through these devices or in their immediate vicinity. Fuses are protective devices or device features that have for some time now been known under various names and in a wide variety of applications. There are known thermal fuses, mechanical fuses, spark gap surge arrestors, varistors, and the like whereby each of these devices is designed to interrupt an unexpected and undesired surge of power that is prone to damage or destroy all or part of an electrical circuit arrangement. Many of the known fuses are designed and used in very specified applications whereby, for instance a mechanical fuse is hardly thought of as having any application in semiconductor devices. The latter type of fuses tends to be created in accordance with known semiconductor processes for obvious reasons of cost and applicability. Not only is a fuse required to be readily integratable with its surrounding electrical environment, it must also provide protection against power surges that can reasonably be expected for that environment. As a result, the designer of semiconductor devices or other electronic components must be well aware of the current and voltage conditions that these components can be exposed to and must, based on this insight, select or design a fusing capability that is best suited for a given application.
Due to the increased device miniaturization and the therefrom following increase in device density, more effort is being dedicated to finding and implementing optimum solutions for the fusing problem. Higher device densities in many cases result in significantly higher current densities, which gives the design of a fusing component a different dimension. Cost and performance are, for any component that makes up part of the highly competitive electronics industry, prime considerations. In view of the pressure that this competitive environment creates and in view of the ever-increasing device packaging density, the challenge of providing adequate device and circuit protection takes on increased urgency and poses an increased challenge.
The parameter that is of key importance to the design of an electrical fuse is the amount of heat that the fuse can dissipate before the electrical continuity that the fuse provides is interrupted. This is particularly true for the electro thermal fuse where the current that is flowing through the fuse heats the fuse. As long as expected and regular current flows through the fuse, the temperature of the fuse and with that the ohmic resistance of the fuse remain relatively low, As soon as the current that flows through the fuse exceeds normal levels, the temperature of the fuse increases eventually causing the fuse to open thereby interrupting the current flow through the fuse. The operational efficiency of the thermal fuse is largely dependent on the characteristics of the metallic conductor that forms the conducting body of the fuse. The power that is dissipated by the fuse is proportional to the square of the current that flows through the fuse. With an increase of the current that flows through the fuse, the dissipated power will significantly increase causing the metallic conductor of the fuse to rapidly increase in temperature, eventually causing the metallic conductor to melt thereby opening the conductive path that leads through the fuse. It is clear that effective operation of the thermal fuse under different conditions of circuit applications depends on strict adherence to critical design parameters of the fuse, most notably the design of the metallic conductor of the fuse. For a number of the thermal fuses, the metallic conductor consists of solder whereby the termination pads of the fuse are made of metallic substances with a melting point that is considerably higher than the melting point of solder. Materials that are typically used for the termination pads are silver, ceramic (or glass) and metal glaze. The solder that is used for the main conductive element of the fuse is connected to the termination pads by the application of heat or energy to the interface between the solder and the termination pad, causing the two interfaces to fuse together. This process may be improved by the use of metallic solder power or a flux in the interface between the solder wire and the termination pad. This enhances the fusing of the two interfaces while not affecting reflow of the solder wire of the fuse.
Solder that is applied for the prior art formation of fuses is susceptible to environmental conditions. To reduce this impact, the termination pad is frequently coated with a thin layer of solder or solder paste before the solder wire is attached. This approach provides a method of preventing arcing or reconnection after the solder wire has been broken, a reconnect that can be caused by creating a relatively large opening at the time that the solder wire of the fuse is interrupted. Oxidation of the solder wire can be prevented by coating the solder wire with a layer of deoxidant material. The solder wire of the fuse can further be protected by encapsulation of this wire including the deoxidant material thereby forming a larger tube that surrounds the solder wire with its deoxidant. The current process of forming a fuse is complex and time consuming and can require as many as in excess of a dozen processing steps. Included in these processing steps are steps of application of solder paste with a subsequent reflow of the solder paste, placement of the solder fuse wire with a reflow to connect the wire to the termination pads, rinsing the reflow material, applying/curing deoxidant in at times more than one repeat cycle, applying an adhesive for a cover for the fuse, placing of the cover of the fuse with a curing of the adhesive and finally encapsulation of the completed fuse. It is obvious that this process is expensive and time consuming and can lead to a serious yield detractor. A better method must therefore be provided for the creation of an effective fuse. The process of the invention provides such a method.
For applications where the levels of current and voltage are relatively low, the art has long used conductive polymers to create fusing capability. Polymers, such as for instance fluoropolymer, are typically used as dielectric materials and as such fall in the same class of materials as silicon dioxide (xe2x80x9coxidexe2x80x9d, doped or undoped) or silicon nitride (xe2x80x9cnitridexe2x80x9d), silicon oxynitride, parylene, polyimide, tetra-ethyl-ortho-silicate (TEOS) based oxides, boro-phosphate-silicate-glass (BPSG), phospho-silicate-glass (PSG), boro-silicate-glass (BSG), oxide-nitride-oxide (ONO), plasma enhanced silicon nitride (PSiNx), and oxynitride. Other polymer materials are polyamide, parylene or teflon, polycarbonate (PC), polysterene (PS), polyoxide (PO) and poly polooxide (PPO), these materials can be used as photoresist applications. In short, polymer compositions can contain silicons, carbons, fluoride, chlorides, and oxygens. It must however be noted that often the term polymer is used to describe chemicals that have been deposited or accumulated (for instance on the sidewalls of an etch chamber). Efforts are also being made to use electrical conductive polymers to connect a chip such as a flip chip or a Ball Grid Array chip to Printed Circuit Board but these efforts are still in the beginning stages.
For applications where a polymer is used in fusing functions, the polymer forms the essence of the body of the fuse while a conductive element is added to the basic polymer component. This conductive element determines the current carrying capability of the fuse and therefore the power level at which the fuse will be interrupted. The conductive polymer that is created in this manner has longitudinal electrodes embedded into the body of the fuse; these electrodes serve to connect the fuse to surrounding circuitry. The resistive element of the fuse is typically surrounded with a protective sleeve that provides electrical isolation and environmental protection for the fuse. The conductive polymer device that is in this manner created can serve to detect and control heat dissipation in particular environments whereby the fuse is interrupted at a point where the temperature of the fuse exceeds a limit.
U.S. Pat. No. 5,777,540 (Dedert et al.) 1998xe2x80x94Encapsulated fuse having a conductive polymer and non-cured deoxidantxe2x80x94shows a fuse structure with conductive polymers interconnects. However, this reference differs from the invention.
U.S. Pat. No. 5,583,321 (Distefano et al.) shows a circuit assembly with conductive polymers (polyacetylene) as a connector.
U.S. Pat. No. 5,925,276 (Batliwalla et al.) discloses a conductive polymer device with a fuse.
U.S. Pat. No. 5,725,995 (Leedy) shows semiconductor probe pins composed of various conductive polymers such as polyacetylene, polythiophene or polypyrrole as examples.
U.S. Pat. No. 5,962,815 (Lan) Antifuse interconnect between two conducting layers of a printed circuit boardxe2x80x94shows a multilayered structure having via holes are filled with a second material having a breakdown voltage less than a breakdown voltage of the first material included in the dielectric layer to form an antifuse. The second material in the via holes can be, for example, a conductive epoxy resin or a polymer loaded with conductive particles.
A principle objective of the invention is to provide a method of forming a fuse using polymeric films as the base material.
Another objective of the invention is to create a fusing function that can be created with a sharply reduced number of processing steps thereby greatly reducing manufacturing complexity and incurred cost for the fusing function.
In accordance with the objectives of the invention a new method is provided for the creation of a fuse. A layer of metal is first deposited, the layer of metal is patterned and etched creating a metal strip that is interrupted by a gap. The fusing function is created in the gap, the interrupted metal strip serves as the connectors to the fuse. A layer of conducting conjugated polymer is deposited over the metal strip and the therein created gap, the polymer is etched back leaving the deposited polymer in the gap between the two metal strips.